1. Field of the Invention
This invention relates to silver-activated zinc sulfide phosphors used as the blue-emitting component in tricolor television picture tubes. More particularly, it relates to a coated luminescent composition wherein the silver-activated zinc sulfide particles form the core and on the surface thereof a uniform adherent coating of a composition represented by the formula, Mg.sub.2 P.sub.2 O.sub.7 :Al.sub.4 (P.sub.2 O.sub.7).sub.3 . xSiO.sub.2 is deposited. The composition does not react with the other components in the photosensitive deposition slurry.
2. Prior Art
Coatings on the surfaces of luminescent phosphors have been used to improve various characteristics of the phosphors and prevent undesired reactions with the components of the photosensitive slurries used to deposit the phosphors on the faceplate of the color picture tube. For example, Sr.sub.2 P.sub.2 O.sub.7 has been used as a coating to improve the adherence of the phosphor on the faceplate and to eliminate the problem known inthe art as "haze". "Haze" as used herein and in the art refers to the non-purity of the color picture resulting from cross-contamination of the three phosphors. In the manufacture of color picture tubes phosphors are deposited step-wise, that is, the complete faceplate is covered with a photosensitive slurry containing one phosphor such as a blue-emitting phosphor. The photosensitive slurry is exposed to light in a predetermined pattern. Thereafter, the faceplate is washed with water and in those areas where polymerization has not occurred the phosphor is removed by the water wash. It has been found that if the phosphor has not been coated with the proper material some of the phosphor particles continue to adhere to the faceplate. For example, as a result of the unwanted adherence some of the area that should contain only the red and green phosphor now contains the blue phosphor. The purity of the colors is thus reduced because of the emission of the undesired phosphors. As is explained in U.S. Pat. No. 3,927,240 a coating of Sr.sub.2 P.sub.2 O.sub.7 resulted in reversing the charge on the surface of the particles thus overcoming the problem with "haze" or cross-contamination.
It has been found, however, that when silver-activated zinc sulfide coated with Sr.sub.2 P.sub.2 O.sub.7, is mixed with the photosensitive solution to form a slurry and the slurry is stored for about 12 hours or longer a reaction occurs with one or more of the components in the photosensitive solution and imperfections occur on the surfaces of the screen. These imperfections are known in the art as "pock marks". When this occurs the faceplate panel is rejected because a bare spot occurs.
It is believed that a blue-emitting cathode ray phosphor that obviates the haze problem and does not adversely react with the components in the photosensitive solution to form surface imperfections is an advancement in the art.